Generally annular clutch facings comprising a fibrous substrate embedded in a matrix of a cross-linked polymeric binder have heretofore been produced by adhering a thermosetting resin such as a phenolic resin together with fillers such as a friction improver, or a combination of a thermosetting resin, a rubber, a vulcanizing agent and a vulcanization accelerator together with fillers to a fibrous substrate comprising asbestos, glass fibers, aramid fibers polyamide fibers or carbon fibers in the form of yarn, roving, tape or fabric, forming the so treated fibrous substrate into a volute or laminate preform, and molding and curing the preform in a mold under heat and pressure to provide the desired clutch facing.
Asbestos-based clutch facings are inexpensive and have good friction properties. Moreover, they are advantageous in that they have excellent operation characteristics since asbestos having a thermosetting resin such as a phenolic resin adhered thereto still retains the softness and resiliency of untreated asbestos. Recently, however, resources of asbestos are becoming exhausted, making asbestos less available on the one hand, evidence has been found indicating that asbestos may be harmful to humans on the other hand. Thus, asbestos-free clutch facings have become of interest in recent years.
For such reasons, glass fiber-based clutch facings have been produced. They have great mechanical strength and excellent wear resistance. However, they pose problems in that when the molded product is finished by polishing or when apertures for mounting the clutch facing on a clutch disc are formed, glass dust is blown up and irritates workers on their skin; and that the heat resistance is not necessarily satisfactory. Another serious problem associated with glass fiber-based clutch facings is a juddering behavior which is felt by a driver as vibration etc. during operation of the clutch facing.
Likewise, aramide fiber-based clutch facings do not necessarily have satisfactory heat resistance.
Japanese Patent Laid-open Publication 61-52432 proposes to produce clutch facings using rock wool as a substrate. However, rock wool-based clutch facings frequently have small tensile strength, although depending upon the manufacturing process, and are generally unsuitable in applications where high rotation break strength is required.
Japanese Patent Publication 60-9528 discloses an asbestos-free friction element for low plane pressure prepared by molding under heat and pressure an admixed friction element-forming composition comprising leather powder, rock wool, vegetable fibers, a friction improver and a thermosetting resin. The disclosed friction element is, however, unsuitable for use as a clutch facing, owing to its poor mechanical strength, small friction coefficient and poor wear resistance.
Japanese Patent Laid-open Publication 57-121080 discloses a friction element comprising metallic powder, slag wool (a kind of rock wool), a binder resin and organic and inorganic fillers. The disclosed friction element has great mechanical strength, and can be used as a friction element for a brake. In a case wherein it is used as a clutch facing, however, it exhibits because of the presence of metallic powder a juddering behavior, which is felt by a driver as vibration, during operation.